Great news this will be a new feature we added recently!
Fundraiser does not have this functionatly to add from their account - however Nonprofit/charity able to do this on behalf of Fundraiser.
How to do this? Login to your charity account, click on 'Fundraiser page' from admin menu and choose the Fundraiser page, scroll to the bottom of the page and you will see a 'Raised Offline' section and you can add the amount into the box provided.
Pleas take note, it will only show on the parent event (if the page is for an event), but not at the charity or Fundraiser level.